With its scofflaw drivers, gridlocked traffic and cobblestoned downtown, Mexico City isn't the most bicycle-friendly place. But residents are being asked to take the risk for Madre Tierra as part of a larger campaign that leaders hope will clean up this 700-year-old metropolis.

"It's now that we need to act, and we are urging other urban leaders around the world to do the same -- to improve quality of life for their citizens and the health of our planet," Mexico City's environmental secretary, Martha Delgado, said Wednesday during a phone interview while visiting Washington to share Mexico City's successes with government leaders and counterparts.

This spring the city launched Ecobici, installing 1,100 bikes at 85 stations throughout the center of the city. In its first three months, 4,000 people have paid $24 for user cards they swipe at a rack to release a bicycle for a half-hour. City officials hope to register 24,000 people in the first year.

With 50,000 trips taken to date through Mexico City's harrowing streets, city officials are delighted with the results: zero accidents, zero thefts.

Introduced in Europe in the 1960s, bike sharing has evolved from a few, informal programs when do-gooders put free bikes out on city streets to high tech systems now used in dozens of cities from Shanghai, China, to Santiago, Chile.

One of the largest is in Paris, where three reported fatalities early on and more than 7,500 thefts led to some serious concerns. But Paris pressed on, opening more bike stations and educating drivers; now studies show the 115,000 bike trips a day have significantly eased traffic.

Ecobici is part of Mexico City's 15-year Plan Verde, a $1 billion-per-year program supported by Mexico's federal government, the World Bank and the United Nations.

The plan includes a new, energy efficient bus system, once-a-week no-drive days for all cars, and subway system improvements. Every Sunday, 16 miles of downtown streets are closed to car traffic, allowing cyclists and pedestrians to safely cruise their city. Parks are staying open later and rooftop gardens being installed.

"This is exactly what we need big cities in emerging economies to be doing," said Mauricio Cardenes, director of Latin America issues at the Brookings Institution.

Here's how the Ecobici system work: Users can check out a bike for 30 minutes at a time. They return it to any of the bike stations, and 10 minutes have to pass before they check out another one. If a bike is kept out longer than a half hour, small fees start to accrue. The bikes have adjustable seats, and they come with lights that go on automatically when ridden and have small racks to hold purses or groceries.

Luis Rodriguez, a downtown office worker, wiped sweat from his forehead as he snapped his Ecobici onto a rack this week.

"The government is doing a good job with this," he said. "It saves me money, gets me to work more quickly and it's healthy for me."

A 15-minute bike commute can burn 11 pounds of fat a year, reducing the risk of heart disease as much as 20 percent, according to SmartBike, which builds Ecobicis for about $700 each in Oslo, Norway. In addition, it says, commuting cyclists are less exposed to air pollution than commuting motorists, and just one person switching from driving to cycling to work for a 6-mile trip each way saves 1.3 metric tons of carbon dioxide emissions a year.

That said, Rodriguez, 35, and every Ecobici user interviewed, said they are gravely concerned about drivers who don't follow rules that allow cyclists to have their own lane. They told of near misses with buses, aggressive drivers leaning on their horns, cars on sidewalks, cars going the wrong way on one-way streets, virtual mayhem at traffic circles.

"Nobody respects the bicyclist," said Gustavo Gonzalez, slipping an Ecobici from a downtown rack. "But I like it. It's a very good program. I wish they'd extend it further."

City leaders do plan to add designated bike lanes, and thousands more bikes as well. They say they have no choice.

The capital and its suburbs face grave ecological challenges. With more than 20 million people and 6 million cars, the metropolis sprawls across a valley surrounded by 16,000-foot peaks that trap contaminants for days. The lakes it was built on have been mostly sucked dry. Pumps run around the clock to control wastewater, but when heavy rains come, nothing stops sewage from washing through the poorer neighborhoods. The city is sinking, too, in some places, as much as a foot a year.

Yet Plan Verde is paying off.

There were 333 days in 1990 that had ozone levels high enough for health warnings. Last year there were 180 days above normal -- still too many, but a marked improvement. Traffic accidents are down 30 percent in areas served by a 4-year-old system that has bus-only lanes on two busy routes, and an estimated 6 percent of people who drove in 2005 are now riding public transport.

With skies changed from mostly brown to mostly blue, the white peaks of the Popocatepetl and Iztaccihuatlcan volcanos can be seen again on many days, rising more than 30 miles from the city's outskirts.

City leaders hope those tangible changes will show other communities that environmental problems can be tackled at the local level.

"We hope that Mexico City will inspire other cities around the world to embrace environmentally sustainable programs," Mayor Marcelo Ebrard said during meetings at Harvard University in November.

Mexico's officials will have an opportunity to show off their results this fall when they host the next major United Nations climate conference.

Metrobike manager Paul DeMaio, who consults from Washington with cities on setting up bike-sharing programs, said Mexico City sets a fine example using Ecobici as one small part of its cleanup plan.

"It's not going to cure the smog," he said. "It's not going to be the silver bullet for making Mexico City into the greenest city in the world, but it's part of the toolbox of measures that local governments can take to reach for those goals."

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